Gear shimmy, a question
 

So, I collected a C206 (late 80s model) from Colombo on a ferry to Belarus. Did three uneventful landings.

Client did next two. First was awful (big hit followed by a series of unbalanced bounces) but he managed to recover. His second was even worse, I had to take control before he groundlooped it (yes, I know, quite a feat in a nosedragger!).

He's not allowed to do landings any more 😀 but both of my subsequent landings have created considerable vibration (shimmy?) during rollout.

Completely unrelated or could there be a correlation?

There could be a correlation. Nose gear shimmy on a Cessna is often related to small bits of play between the moving parts on the nose gear. The bounces and partial groundloop may have caused this play to increase. It may also have caused uneven wear on the nosewheel tyre, which can also lead to shimmy.

What Jhiemnga said...…...Loose parts/excessive play in the strut/scissors link bushings, bolts, etc., or a bad worn out shimmy damper. Or, badly worn or out of balance nose tire. Any or all could cause nosewheel shimmy. Sounds like when you landed the plane, you were better at holding weight off the nose during the rollout than the other fliers. Often, as instructor, when the flying student allows the nose to "wheelbarrow" too much during landing, a quick pull aft on the yoke to reduce weight on the nose will stop the vibration. Then at slow taxi speeds, the shimmy may not be evident. A 206 is quite a bit heavier in front, as is a 182, especially if the pilot is transitioning from, say, a lighter-handling 172. I'd have a mechanic check out the nose gear for wear and/or (hopefully not!) more extensive damage, firewalls have been known to get bent from nosewheel-first landings in 182s and 206s.

The two foregoing posts are spot on, all of the mechanism of the nose strut must be a very close fit - zero sloppy movement. Ive rebuilt many of these struts. If there are any loose points in the mechanism, a gentle hand ferry flight for maintenance only. If there is a nosewheel fairing, remove it, they worsen shimmy. Inflate the nose tire to the highest allowable pressure, and have it balanced when possible.

If the aircraft has any wing mods (STOL kit, particularly Robertson), or Flint Tips, these make the elevator even less effective at raising the nose.

If you're training someone on a 206, insist that the nosewheel be held light for all ground operations - full nose up elevator during any rolling slower than flying speeds. No tolerance for letting the nose be heavy. Certainly no tolerance for anything like a nosewheel fist landing! If the candidate pilot is having trouble, have them land with no more than 20 flap until they get used to it. All taxiing should be done with 15 degrees of flap extended, and full up elevator control. The changed downwash over the tail will assist in keeping the loads off the nosewheel. 206's are particularly vulnerable to very expensive nose structure damage from mis handling.

I was flying a very low time Cessna T206H the other day, including many circuits. I experimented with shimmy avoidance in mind. In this aircraft, with full fuel, and two of us in the front seats, it was easily possible to raise the nosewheel off the surface at 25 MPH indicated, and hold it there with no difficulty, controlling the takeoff form there from the mains only. Extended flaps and partial power, with full up elevator was used, and then right rudder to keep it straight with the application of the rest of the power. Similarly, landing, there was no problem holding the nosewheel off to happily slow speeds, while controlling direction with rudder only.

During the same customer service, I also flew a few circuits in a Cessna R172K. Prior to the flying, I found that the nose gear steering collar and shimmy damper attachments were as loose as I would tolerate. Similarly, I held the nose light, and the flying was fine. I reported to the maintainer my recommendation that the oleo steering be serviced.